There is increasing interest by government agencies and the general public in protecting individuals against the harmful effects of toxic substances. In fact, personal monitoring of exposure to specific hazardous substances in the environment is required for many designated materials by government regulations. It is desirable that such devices indicate when exposure to a specific hazardous substance has reached a selected time-weighted-average value. Devices for monitoring exposure to a hazardous substance based on a visually observable change of color of a dye or color precursor incorporated therein are currently available.
Colorimetric indicators have been disclosed as useful in filtering devices, such as respirators. U.S. Pat. No. 1,537,519 discloses use of an indicator strip, such as litmus paper, in a window-type respirator. U.S. Pat. No. 4,155,358 discloses a disposable valveless chemical cartridge respirator for filtration of vinyl chloride monomer which includes a visually observable end of service life indicator. U.S. Pat. No. 4,154,586 (and related German and British Pat. Nos. 2,758,603 and 1,554,542, respectively) discloses a respirator cartridge providing a visual means for indicating organic vapor/gas hazards and incorporates a catalytic agent for enhancing activation and reaction of the indicator agent. U.S. Pat. No. 3,966,440 discloses a canister or cartridge which incorporates a colorimetric indicator for use in indicating the impending exhaustion of a toxic gas filtering device. These patents generally relate to devices with granular silica gel or alumina packed cartridges.
U.S. Pat. No. 3,350,175 relates to a finely divided non-porous inorganic carrier such as glass beads or powdered glass, the particle size being 40 to 100 mesh, on which is coated a powdered inorganic absorbent, such as clay, diatomaceous earth, or bauxite, impregnated with a colorimetric reagent. The free-flowing, discrete, coated particles of the patent are packed as an elongated bed in a transparent tube. Also relating to particles packed in a tube is U.S. Pat. No. 3,068,073 which discloses a method and reagent for detection and quantitative determination of carbon dioxide. The gas to be tested is passed through an elongated bed packed with alumina, between 16 and 200 mesh size, carrying thymol blue or thymol blue and a base.
It is known in the art to use an adhesive to secure absorptive materials to substrates. U.S. Pat. No. 3,672,845 discloses an indicator testing device wherein an indicator is incorporated in hydrophilic absorbent particulate material such as aluminum oxide, silica gel, cellulose, or vermiculite, and secured to a sheet by means of a suitable adhesive. U.S. Pat. No. 3,552,929 relates to a device for detecting halide ion concentration in fluids and discloses use of an adhesive to hold an indicator impregnated substrate, e.g., filter paper, to a backing, e.g., polyester or glass.
Heftmann, Erich, Chromatography, 3rd edition, Van Nostrand Reinhold Co., New York (1967), pp. 164-183, relates to thin-layer chromatography and discloses, for example, siliceous materials, cellulosics, polyamides, and dextran gels as components of coating materials for thin-layer chromatography plates.
Monitoring of personal exposure to hazardous materials is the subject of a number of studies of which the following are examples: Natusch, Sewell and Tanner, "Determination of H.sub.2 S in Air--An Assessment of Impregnated Paper Tape Methods", Analytical Chemistry, volume 46, page 3 (1974); Schnakenberg, "A Passive Personal Sampler for Nitrogen Dioxide", Bureau of Mines Technical Progress Report 95 (1976); Ray, Carroll and Armstrong, "Evaluation of Small Color-Changing Carbon Monoxide Dosimeters," Bureau of Mines Rep. Invest. (1975); Palmer, "Personal Samplers for CO, NO and NO.sub.2 in Air", Bureau of Mines Report OFR 92-77 (1977) and Nichols, "Reactive Tapes for Automatic Environmental Analysis, Personal Vapor Monitoring Badges for Industrial Workers", National Science Foundation Report NSF/RA-780039 (1978).
Indicators in sheet form have been utilized in the prior art. The method of coating the carrier and dye material onto a film backing employs binders of soluble organic polymers or organic polymers dispersed in an aqueous phase. These organic polymer binding systems suffer serious drawbacks and limitations. In order to bind the carrier particles so that they will adhere, it is necessary to coat the particles themselves with resin. This coating lowers the ability of the carrier particles to adsorb the dye. In addition, the hazardous gas must penetrate a layer of resin polymer in order to reach the adsorbed dye. This coating also serves to slow the rate of reaction. Thus, resin binders tend to create exactly the opposite effect than intended.
In addition to the above limitations, there is the interaction of certain reactants with organic compounds. Chemicals such as permanganate salts, chromate salts, strong acids and bases and other reactive materials useful in dye and dye precursor formulations are often not stable in contact with organic materials and thus cannot be used with organic resin binders.
The colorimetric indicators of the present invention overcome the above-mentioned limitations of the organic resin binders included in the prior art compositions. The present invention provides colorimetric indicators comprising certain clay mineral binders, which clays possess properties of high porosity, large specific surface area, superior adhesiveness, stability towards many dyes and dye precursors, ready availability and economy.
Clays have been used for numerous purposes, many of which depend on their adsorptive ability. They have been used as sweeping compounds, as drilling muds in oil drilling, and in ceramics. There are many references to their use as dye acceptors or pigmented particles. See, for example, R. W. Grimshaw, The Chemistry and Physics of Clay, Wiley-Interscience, London (1971) pp. 120-124, 140-148.